1. Field of the Invention
The present invention relates to an optical recording/reproduction apparatus that optically records information on an information storage medium using a light source such as a laser and converts the information recorded on the information storage medium to a signal and reproduces it, or to an optical disk apparatus such as an optical reproduction apparatus that converts information pre-recorded on an information storage medium to a signal and reproduces it, and more particularly, to a control apparatus and an optical disk apparatus for performing control so that optical beam spots scan tracks correctly.
2. Description of the Related Art
As an example of a tracking control apparatus of a conventional optical disk apparatus, one that corrects lens shift so that an offset of a phase difference tracking error signal becomes zero is known.
FIG. 28 is a block diagram schematically showing a configuration of a conventional optical disk apparatus disclosed in Japanese Patent Publication No. 2000-315327. In the optical disk apparatus shown in FIG. 28, an optical pickup 20 is provided with a laser light-emitting element (not shown), a converging lens 22 and an actuator 23, and converges and irradiates an optical beam 21 onto an optical disk 10. The optical pickup 20 is further provided with an optical detector 24 which is divided into four detection sections A to D and the optical detector 24 detects a reflected beam 21′ reflected by an information recording plane of the optical disk 10. The outputs of the detection sections A to D of the optical detector 24 are input to a signal generation circuit 30.
The signal generation circuit 30 is provided with a phase difference adjusting circuits 31a and 31b and adjusts a phase difference (referred to as a “tangential phase difference”) in the circumferential (tangential) direction of the disk with respect to the output signals of the detection sections A and B input from the optical detector 24. This makes it possible to remove an offset caused by a phase difference produced between the outputs of the detection, sections A to D. Addition circuits 32a and 32b generate an addition signal (A+D) and an addition signal (B+C) resulting from adding up the outputs of the detection sections positioned at opposite angles of the optical detector 24 and a phase difference detection circuit 33 detects these phase differences. The output of the phase difference detection circuit 33 which is output through an LPF 34 becomes a phase difference tracking error signal (referred to as “DPDTE”).
A digital signal processor (referred to as “DSP”) 50 includes an offset adjusting section 52, a tracking control section 53, an offset measuring section 61, a lens shift correction section 62, an A/D converter 51 and a D/A converter 54.
The A/D converter 51 converts the DPDTE to a digital signal. The offset adjusting section 52 adds an offset for tracking control to a digital signal of the DPDTE. Furthermore, the tracking control section 53 generates a tracking drive value by carrying out a filter calculation for phase compensation and low-frequency compensation on the DPDTE digital signal. The tracking drive value generated is converted to an analog signal by the D/A converter 54 again and output to a drive circuit 91 as a tracking drive signal. The drive circuit 91 carries out current amplification on the tracking drive signal, drives an actuator 23 with a built-in optical pickup and carries out tracking control.
Then, a method of correcting lens shifts according to a conventional technology will be explained. With the optical pickup 20, the center of the converging lens 22 may shift from a set position due to mounting errors of optical parts including the converging lens 22 and optical detector 24, drooping of the converging lens caused by holding the apparatus in a vertical position or deviation of optical axis of a beam emitted from a laser. In this case, the reflected beam forms an image deviated from the center of the optical detector 24. In the following explanation, this state is referred to as a “lens shift.” This lens shift is a shift of the position of the converging lens 22 or optical axis from the center of the optical detector 24 generated inside the optical pickup.
FIG. 29A to 29C show phase difference tracking error signals output from the optical pickup in various lens shift conditions when no tracking control is performed. A tangential phase difference caused by a lens shift is observed in the signals shown in FIG. 29A and 29C. More specifically, the signals shown in FIG. 29A and 29C are generated when the converging lens shifts from the center of a track toward the inner radius side and the outer radius side of the disk by about 300 μm. On the other hand, the signal shown in FIG. 29B is obtained when there is no shift in the converging lens.
When there is a tangential phase difference due to a lens shift as shown in FIG. 29A and 29C, if the converging lens is shifted through tracking control, the symmetry of a phase difference tracking error signal deteriorates and an offset is generated. A relationship between this DPDTE offset and lens shift is shown in FIG. 30. In FIG. 30, the horizontal axis shows the position of the converging lens and the vertical axis shows an offset value of the DPDTE.
As shown in FIG. 30, the position of the converging lens and offset value of the DPDTE show a linear relationship near an optimum lens position. By detecting this DPDTE offset and applying an offset to the tracking drive value so that the DPDTE offset becomes 0, it is possible to shift the position of the converging lens 22 and correct the lens shift.
Then, with reference to FIG. 28, a procedure for correcting a lens shift will be explained. The adjustments of the phase difference adjusting circuits 31a and 31b of the signal generation circuit 30 are shifted to create a condition in which a tangential phase difference has occurred. If a lens shift has occurred, a DPDTE offset occurs, and therefore the DPDTE offset is measured using the offset measuring section 61.
The lens shift correction section 62 adds this offset to the output value of the tracking control section. The drive circuit 91 moves the converging lens 22 based on the output value of the tracking control section to which this offset is applied. Here, moving the converging lens 22 using the linear relationship shown in FIG. 30 so that the DPDTE offset detected by the offset measuring section 61 becomes zero makes it possible to reduce the lens shift to zero.
For example, as in the case of a DVD-RAM disk, when addresses shifted toward the inner radius and outer radius by ½ track each with respect to a track are reproduced, if a lens shift occurs due to eccentricity or deviation of the optical axis, etc., balances of RF signals at the address section are lost. This prevents a gate signal for detecting and separating the address section from being generated or reduces the amplitude of the RF signal at the address section, deteriorates a signal to noise (S/N) ratio and thereby produces a problem of failing to reproduce address information correctly.
Solving this problem using the lens shift correction method in the above-described conventional optical disk apparatus requires a mechanism (detector) and an adjusting circuit for adjusting the tangential phase difference as shown in the signal generation circuit in FIG. 28. For this reason, there are problems that it is difficult to reduce the cost of the optical disk apparatus or reduce the size of the optical pickup.
The present invention has been implemented in view of the above-described problems and it is an object of the present invention to provide a tracking control apparatus and optical disk apparatus capable of reducing a lens shift to zero even for an apparatus which uses no phase difference tracking error signal and includes no phase difference adjusting circuit or an optical pickup simply divided into optical detectors.